This invention relates to the field of printing. In particular, this invention is drawn to digital halftoning of continuous tone images.
Rendering of continuous tone images such as photographs typically requires a halftoning process to convert the continuous tones into discrete tone levels that can be realized through a printing device such as a laser printer. Sophisticated halftoning processes are designed to create a halftoned image that is reproducible on the discrete rendering device while minimizing visually perceptible differences between the original image and the rendered image.
One halftoning technique used in laser printer applications is referred to as clustered dot dithering. A threshold or halftone matrix is used to halftone the image. Generally, a halftone value is computed for each selected pixel as a function of the tone value of the selected pixel and the value at the corresponding location in the halftone matrix. The pattern and size of the final halftone dot structures are determined by the entries and size of the halftone matrix.
Although large array sizes can be designed as high spatial frequency screens by selecting the appropriate values for the matrix elements, the larger matrix sizes are typically used to realize low spatial frequency screens. Low spatial frequency screens are efficient and accurate for large areas of substantially constant or slowly varying tone, but the low spatial frequency screens tend to smooth or blur fine detail such as lines and edges. The low spatial frequency screens tend to reproduce smooth tone regions well at the expense of errors or loss of detail in the busy areas of the reproduced image.
The use of high spatial frequency screens can bring out detail in perceptually busy areas of the image such that detail is substantially maintained in the halftoned image. The high spatial frequency screens, however, tend to result in halftoned images that have perceptual artifacts when compared with the original image in the areas of substantially constant or slowly varying tone. The use of high spatial frequency screens tends to result in greater detail in busy areas at the expense of introducing errors in the smooth areas of the image such as area fill and non-edge regions of a photograph.
Thus one disadvantage with conventional digital halftoning processes is that choosing exclusively between a threshold matrix having a high spatial frequency and one with a low spatial frequency necessarily trades detail for tonal quality.
In view of limitations of known systems and methods, methods and apparatus for adaptive halftoning to accommodate images having a mix of high and low visual activity are provided. Accommodations for white gap reduction and variable darkening are provided for print engine imperfections.
In one embodiment, a halftoning method includes the step of thresholding a selected pixel of a source image to generate a first dithered output signal, L. The selected pixel is also thresholded to generate a second dithered output signal, S. A weighted combination of the first and second dithered output signals is generated in accordance with at least one of an edge activity indicator parameter (EAI) and a mix suppression parameter (xcex7).
The edge activity indicator is indicative of the frequency of change within a selected region containing the selected pixel. The combination step weights the first dithered output signal (resulting from thresholding using a low spatial frequency) more favorably when the EAI indicates relatively little changes indicative of larger areas of non-varying or slowly varying tonal values in the selected region. The combination step weights the second dithered output signal (resulting from thresholding using a high spatial frequency) more favorably when the EAI indicates greater amounts of change indicative of edges and fine lines within the selected region of the source image.
The mix suppression parameter is used to suppress the weighting of the second dithered output signal relative to the first dithered output signal on the light side of an edge as well as to preserve the sharpness of anti-aliased edges.
The second dithered output signal (i.e., S, resulting from thresholding at a higher spatial frequency than that used for the first dithered output signal) may be boosted to the value V using a variable darkening parameter, xcex3, to accommodate isolated dot gain problems before being combined with the first dithered output signal, L. Alternatively, variable darkening can be eliminated by setting 65  to 0.
An edge indicator (EI) may be used to compensate for more favorable dot development along edges between a darker tone region and an almost white region. The EI parameter is particularly suitable for handling white gap reduction due to printer anomalies.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.